Well, you're drawing the graph. Each item in the graph already has an X value. The actual GUI for the scrollbar I am not going to help you with right now, but all you would do is multiply the X value of any given element in the graph by the amount in the scrollbar.
And for sliding it, you just offset the value by the reading from the scrollbar. So add (or minus, depending on how you are reading from the scrollbar) the X scrollbar value and the X from the element and you're good to go.